Rubber compositions are used in a variety of applications, including tire components such as treads and sidewalls, hoses, belts, footwear components, vibration isolation devices and bladders. While the particular rubber compositions used in each of these applications vary widely in their physical properties, one attribute remains the same—their color. Most rubber compositions are black. Furthermore, most rubber compositions eventually become discolored due to heat, light, ozone, etc. This is particularly true for rubbers used in stressful, demanding applications such as tire treads and sidewalls.
Practitioners in this field will point to the presence of the reinforcing filler “carbon black” as a prime reason that most rubbers are black. While this is true, carbon black is not the only factor. In fact, a wide variety of other fillers, curatives, antidegradants, oils and the rubbers themselves can all result in a dark color that is essentially impossible to pigment. In particular, naphthenic, aromatic, and paraffinic oils and resins can often cause discoloration and are undesirable. This is evident in compositions where carbon black has been replaced with a silica filler and the rubber is still discolored. For example, European Patent 0 682 071 B1 discloses a silica reinforced tire tread which, due to the presence of the aromatic processing aid, coupling agent, antidegradants and a sulfur curative system, will still be dark in color. In fact, it is uncertain how many of the ingredients present in the rubber composition would have to be changed to produce a colorable composition.
Resins and oils (or “processing aids”) such as naphthenic, paraffinic, and aliphatic resins may be used to improve the processability of elastomeric compounds. However, increased processability in the presence of oils and resins comes at the price of a loss of air impermeability and an increase in undesirable color, among other undesirable effects of various other properties.
Polybutene and paraffinic-type processing oils have been disclosed in U.S. Pat. No. 4,279,284 to Spadone, U.S. Pat. No. 5,964,969 to Sandstrom et al. and EP 0 314 416 to Mohammed. A paraffinic-type processing oil is disclosed in U.S. Pat. No. 5,631,316 to Costemalle et al. Also, WO 94/01295 to Gursky et al. discloses the use of petroleum waxes and naphthenic oils and resins in a rubber composition for tire sidewalls, and U.S. Ser. No. 09/691,764, filed Oct. 18, 2000 (assigned to the assignee of the present invention) to Waddell et al. discloses a colorable rubber compositions. Other disclosures of processing oil or resin-containing elastomeric or adhesive compositions include U.S. Pat. Nos. 5,005,625, 5,013,793, 5,162,409, 5,178,702, 5,234,987, 5,234,987, 5,242,727, 5,397,832, 5,733,621, 5,755,899, EP 0 682 071 A1, EP 0376 558B1, WO 92/16587, and JP11005874, JP05179068A and JO3028244. None of these disclosures solves the problem of improving processability of elastomeric compositions useful for tires, air barriers, etc, while maintaining or improving the air impermeability of those compositions. Further, these materials may not be suitable for applications requiring high abrasion resistance while maintaining the colorability of the composition. Such is the case for, example, tire sidewalls and shoe soles.
White sidewalls on tires are a form of colorable rubber. The white color is achieved by using fillers such as silica, clay, talc and carbonates instead of carbon black and adding titanium dioxide as a whitening pigment. However, the white color comes with a price. The fillers are more fragile than carbon black and result in a weak rubber composition that does not reinforce the tire.
In applications such as shoe soles and tire treads where a large amount of a filler such as silica is used, it is desirable to maintain and adequately adjust certain physical properties such as the processability of the rubber, the cure rate, and final cure characteristics such as traction and abrasion resistance. In particular, abrasion resistance is desirable for colorable rubber compositions in, for example, tire sidewalls where there is a need for resistance to curb-scuffing. Also, in shoe soles and other shoe applications there is a need for an elastomeric composition that has long wear. An object of the present invention it to provide a colorable elastomeric composition that has improved abrasion resistance and other improved physical characteristics, while maintaining the colorability and transparency of the composition.